Modified rule to support an empty alt attribute

Images may have an empty alt attribute if it's used for cosmetic purposes.

From http://www.w3.org/TR/html4/struct/objects.html#adef-alt :

"Do not specify irrelevant alternate text when including images intended to format a page, for instance, alt="red ball" would be inappropriate for an image that adds a red ball for decorating a heading or paragraph. In such cases, the alternate text should be the empty string ("")."